This invention relates to an electrochemical cell, and, more particularly, to the structure of the electrode leads of such a cell wherein the terminals are at the same end of the cell.
Rechargeable electrochemical cells are electrochemical energy storage devices for storing and retaining an electrical charge and later delivering that charge as useful power. The energy storage cell may be recharged after it has delivered the useful power, leading to a succession of charging and discharging cycles. Familiar examples of the rechargeable energy storage cell are the lead-acid battery used in automobiles and the nickel-cadmium battery used in various portable electronic devices. Another type of electrochemical cell having a greater storage capacity for its weight is the nickel oxide/pressurized hydrogen energy storage cell, an important type of which is commonly called the nickel-hydrogen cell and is used in spacecraft applications.
The nickel-hydrogen battery includes a series of active plate sets which store an electrical charge electrochemically and later deliver that charge as a useful current. The active plate sets are packaged within a hermetic pressure vessel that contains the plate sets and the hydrogen gas that is an essential active component of the energy storage battery. Each plate set includes a positive electrode, a negative electrode, and a separator between the two electrodes, all soaked with an electrolyte. In a typical nickel-hydrogen cell, a number of plate sets are supported on a core under a compressive loading, with a gas screen between each plate set and with electrical connector leads extending to each electrode of each plate set. The gas screen provides a gas channel from the hydrogen electrode to the gas space outside the stack. A single nickel-hydrogen cell delivers current at about 1.3 volts, and a number of the cells are usually electrically interconnected in series as a battery to produce current at the voltage required by the systems of the spacecraft.
The nickel-hydrogen cell may be configured such that its positive terminal and its negative terminal are at opposite ends of the pressure vessel or at the same end of the pressure vessel. When the terminals are at the same end of the pressure vessel, it has been observed that the individual plate sets may wear out at different rates over the life of the cell. This uneven performance has been traced to the different electrical resistances of the electrode leads. That is, the electrode leads extending to the plate sets nearest to the terminals are shorter than the electrode leads extending to the plate sets furthest from the terminals. The electrical resistance of the electrode leads varies according to their length, leading to a greater electrical resistance for the longer leads than for the shorter leads, which in turn causes the uneven performance of the individual plate sets.
Two approaches have been proposed for overcoming this problem. In one, the electrode leads are made larger in cross section, so that their resistivity is smaller and there is consequently less variation in total resistance for the longer and the shorter electrode leads. This approach results in an undesirable increase in weight for the electrode leads. In a second approach, the shorter electrode leads are made of smaller-cross-section electrical conductor material than the longer electrode leads, resulting in a closer matching of the total resistance of the electrode leads. The second approach results in added complexity in manufacturing due to the necessity of stocking a number of different sizes of electrode leads, and also has not been found to be totally successful in overcoming the problem in practical applications.
There is a need for an improved approach to avoiding uneven performance in the plate sets of nickel-hydrogen and other types of electrochemical cells whose terminals are at one end of the cell. The present invention fulfills this need, and further provides related advantages.
The present invention provides an improved electrochemical cell whose electrode leads are graded to provide substantially constant electrical resistance regardless of the lengths of the electrode leads to the various plate sets. The performance of the various electrode lead/plate sets is thereby equalized. The approach is easily implemented in the manufacturing of the electrochemical cell, and does not require the stocking of a large variety of leads of different cross-sectional areas.
In accordance with the invention, an electrochemical cell such as a nickel-hydrogen cell comprises a terminal set, including a positive terminal and a negative terminal, and a cell including at least two plate sets. Each plate set includes a positive electrode, a negative electrode, and an electrolyte contained in a separator therebetween. There is an electrode lead set for each respective plate set. Each electrode lead set includes a metallic conductor lead comprising a positive metallic conductor lead extending between the positive terminal and the respective positive electrode, and a negative metallic conductor lead extending between the negative terminal and the respective negative electrode. The various positive metallic conductor leads are usually of different lengths for the various plate sets, and the various negative metallic conductor leads are usually of different lengths for the various plate sets. A cross-sectional area of the metallic conductor lead varies along a length of the metallic conductor lead. Preferably, the cross-sectional areas of the metallic conductors are selected such that the electrical resistance of each metallic conductor lead is substantially a constant value.
In one embodiment, the cross-sectional areas of the positive metallic conductors are selected such that the electrical resistance of each positive metallic conductor lead is substantially a constant value. The cross-sectional areas of the negative metallic conductors are selected such that the electrical resistance of each negative metallic conductor lead is substantially a constant value.
The cross-sectional areas are preferably graded according to regions along the length of the electrode leads. There may be a first region having a first cross-sectional area, and a second region having a second cross-sectional area, with optionally a transition region between the first region and the second region. The region of smaller cross-sectional area has a relatively higher resistivity per unit length, while the region of larger cross-sectional area has a relatively lower resistivity per unit length. By selecting the relative cross sectional areas and the relative lengths of each region, the overall electrical resistance of the electrode lead may be precisely established. In the approach of greatest interest to the inventors, a single first region area and a single second region area are used for each of the electrode leads. All of the leads for an electrochemical cell may be cut, as by punching, from a single basic flat stock material whose as-supplied cross-sectional area is the first-region cross-sectional area. The second region is formed by cutting or punching a selected length to a uniformly narrowed second region cross-sectional area. This minimizes stocking requirements and the potential for error in manufacturing operations.
The present approach permits the electrical resistances of the leads to the various plate sets to be precisely established. The result is that the plate sets wear out at a more uniform rate, without the variations in lead resistance found in conventional approaches. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.